Alph - A Little Process Helper

This is the sixth part of the Alph introduction tutorial. If you did not go through the previous parts, you might want to start with them first, but a completed tutorial 5 case is available.

Alph tutorial pages:

- 1 - Getting started and creating a fluid
- 2 - Hydrate and phase envelope tools
- 3 - Fluid interactions and formulas
- 4 - Mixing and component splitting
- 5 - Distillation Tower
- 6 - Simple refrigeration cycle
- 7 - Function Solver
- 8 - Material Recycles

To some extent this exercise is gratuitous, but the calculation of a simple refrigeration cycle was my favorite magic trick in the early days of Hysim and I just can't resist duplicating it on my phone.

Previously we worked through this simple gas plant flow sheet.

Now we shall add the propane refrigeration cycle that will chill the chiller:

This is about the simplest refrigeration loop possible. Cooling the process fluid boils the propane in the chiller. The vapour is sent to a compressor, where the pressure is raised. The high pressure propane is now condensed, usually with an air exchanger, and the resulting liquid flashed through a valve to produce a cold mixed phase fluid that is fed back to the chiller. The net effect is to pump heat from the chiller to the condensor.

Continuing from where we left off in part five, create a new fluid with the **Add Fluid** button on the diagram.

Name the fluid "c3comp" and then leave a space and follow the name with "Propane to Compressor" as a description.

In the temperature field, enter:

@lts - $dt

Now switch the **P** button to **Vf** by successive taps. Enter 1 for the vapour fraction since the propane boiling off of the chiller will be at its dew point.

If the flow and composition fields have been filled in, clear them and enter the composition as pure propane.

You can use the composition input page (tap the blue icon to the right of the **X** field) or you can just enter:

[ 0,0,1,0,0,0,0,0 ]

directly in the field.

Tap **Bulk** to see the result

Alph has calculated the dew point pressure of the vapour. This will be the pressure of the propane leaving the chiller.

Return to the diagram and create another fluid.

Label this "c3jt Propane to the JT valve" as it will represent the fluid leaving the condenser and entering the Joule Thompson valve. Assuming no sub-cooling, this fluid will be at its bubble point, so we can assign the vapour fraction a value of 0. Picking 45 C as a reasonable air cooling temperature and setting the composition to be the same as fluid going to the compressor, i.e. all propane, allows Alph to determine the bubble point pressure.

Tap **Bulk** to check the value.

All we need now is the flow. Since the valve is isenthalpic, we know that the propane going to the chiller will have the same specific enthalpy as this fluid. We also know the specific enthalpy of the propane leaving the chiller - our c3comp fluid, so a heat balance can be used to calculate the flow through the chiller.

Open the formula editor for **F**.

The chiller duty is just the chiller feed energy flow, minus the lts energy flow. Dividing that by the enthalpy difference in the propane going through the chiller gives us a flow rate.

(@chillfeed.Q - @lts.Q) /

(@c3comp.H - @c3jt.H)

Switch back to the **Bulk** view to confirm the flow has been calculated.

Since this flow will be the same throughout the propane loop, you can **return to the c3comp fluid and set its flow to @c3jt**.

To finish things off, we will model the propane compressor. An ideal compressor is just an isentropic flash and even with an adiabatic efficiency, the calculation can easily be done with formulas, but like the mixer, a convenient compressor / expander tool is provided.

Go to the diagram and create a new compressor.

Name the compressor "c3comp" (yes a fluid and a tool can have the same name) and describe it as "Propane Compressor".

Set the inlet fluid to be "c3comp" and the pressure to the outlet pressure of the condenser (**c3jt**), plus its pressure drop (**%dp**). Finally set the efficiency to be **75**. The compressor actually uses fractional efficiencies, but if the efficiency value is greater than 1.0, it is automatically divided by 100.0.

At this point the tool will calculate the outlet enthalpy and display it at the bottom of the page. If the feed flow is known, the required compressor power will also be displayed. Like the mixer, outlet values for **H**, **F**, **Q**, **X** and **P** can be referenced from formulas. Also **E** can be used to retrieve the efficiency and **power** to retrieve the required compressor power.

Return to the diagram and create a new fluid to represent the inlet to the condenser.

Name the fluid "c3cond" and describe it as "Propane to condenser". Obviously it is also the fluid leaving the compressor. Select pressure and enthalpy as the two independent variables and then enter **#c3comp** in the pressure field. The result should be filled in with the same thing, which is what we want

Tap the **Bulk** button to check the results.

That's it. The propane loop is completely defined. For completeness you might want to create a c3chill fluid as the propane feed to the chiller, but it won't really add anything to the solution, except maybe properties for the mechanical design of the chiller.

In the next tutorial section, we will look at how to use the **Function Solver** tool to deal with iterative problems, such as adjusting values to meet certain specifications. In a later section we will also look at using it to solve recycle problems.